Even longtime swimming coaches profess to being baffled by the more than three dozen world records broken in the last 18 months in pools around the world. They wonder how to fully explain such a sudden and widespread explosion of speed in a sport contested since the first Olympics more than a century ago.

The answer, they say, cannot lie solely in the latest high-tech swimsuits introduced amid a swirl of controversy this winter, because the world-record smashing began at last year's world championships -- long before the newest of the newfangled apparel came out.

Swimmers, coaches and scientists say it is impossible to pinpoint one explanation. They cite many contributing factors, ranging from professional training groups that have sprouted across the United States to greater access to underwater cameras and other advanced technology.

But some say the most significant breakthrough has been a revival of a swimming maneuver developed more than 70 years ago by one of the physicists who worked on the atomic bomb.

Though utilized for decades, the underwater dolphin kick had not been fully exploited by the swimming mainstream until Olympic megastar Michael Phelps and a few other stars began polishing it -- and crushing other swimmers with it -- in recent years. Some say the revival has caused a quiet sensation that has been largely drowned out by the reaction to the suits, whose tightness, futuristic fabric and seam-free design make swimmers sleeker and more streamlined.

It is the use of the dolphin kick, coaches point out, that keeps swimmers where they can best take advantage of whatever advantages the suits offer: underwater.

"You cannot succeed without this skill," said Mark Schubert, the head coach and general manager of USA Swimming's national team.

"It's a weapon," said Jonty Skinner, the performance science director for the U.S. national team.

"It's been a quantum-leap difference," said Phelps's longtime coach, Bob Bowman. "Michael's going 13 meters underwater [using the kick] instead of five. That was what he did that Ian Thorpe didn't."

Bowman was referring to Phelps's demolition of Thorpe's world record in the 200-meter freestyle last year, an achievement that stunned fans at the world championships in Melbourne, Australia. The mark set by the now-retired Thorpe, the greatest swimmer of his era, had been considered virtually untouchable before Phelps's swim.

But Bowman said the difference in Phelps's record race (1 minute 43.86 seconds) and Thorpe's 2001 effort (1:44.06) was plain: Phelps stayed underwater longer off the turns, executing the undulating motion with his entire body that is designed to mimic a dolphin's use of its flipper. It wasn't that Thorpe did not use the dolphin kick. All elite freestylers have for years, rather than the old-fashioned flutter kick. But Thorpe came to the surface earlier throughout his race, dolphin-kicking less and relying more on his freestyle stroke.

The problem for Thorpe? When executed properly, experts say, the underwater dolphin kick is faster than any stroke except a full-out freestyle sprint over 50 meters.